Sensing apparatus for reducing sheet detection and registration errors

Abstract: An apparatus (100) for sensing the presence or an edge of a sheet, including a light emitter (103), a set of reflectors (101,102,106,107) and at least one light detecting member. A light beam is reflected between the emitter (103) and detector (110,111) in a repeating wave form so as to intersect a sheet path; interruption of the light beam indicates the presence of a sheet. False indications of the presence, absence or true edge of the sheet such as may be caused by sheet holes or edge irregularities are avoided by the multiple passes of a beam of light across the path of the sheet. A continuous cleaning action of the sensor (110,111) may be provided by the movement of the sheet over the light emitters (103), light detectors (110,111) and/or reflectors (101,102,106,107). The sheet sensors are useful in detecting and/or correcting sheet skew, misregistration and other potential sheet handling problems. Through the use of optic fibers (Fig. 5), wires or other means, the light emitter(s) and light detectors that may service a network of sheet sensors positioned throughout a copying or printing machine (Fig. 11) which may be centralized and otherwise assembled in efficient modularized configurations.

Description: This invention relates generally to sheet handling devices, and more particularly concerns an apparatus and method for correctly sensing the leading edge, the trailing edge and/or presence of a sheet so as to avoid sheet handling errors that might otherwise occur as a result of pre-drilled sheet holes, tears and other variances in a sheet surface, or that might occur as a result of sheet skew, misalignment, misregistration, paper jams or other sheet handling situations. Xerographic reproduction and printing machines utilize sheet handling devices which often incorporate a number of sensors to monitor sheet presence, edge location, dimensions, alignment, registration, jamming and other conditions relating to sheets passing through such machines. Extensive arrays of such sensors are being incorporated into technologically advanced machines to increase their capabilities and reliability, and to prevent hazardous conditions and/or damage to machines that may be caused by sheet jamming or other problems. Perhaps most notably, sheet sensors can eliminate numerous preparation and instruction input tasks formerly required of operators. The employment of compact and reliable single and multi-function sensors that can be used more than one location in a machine can further enhance the capabilities of such machines, while offering important manufacturing and repair cost advantages and cost savings. Sheet presence, alignment and edge detection systems have heretofore existed which make use of light emitters and receptors. An important drawback to many such sensors is their inability to correctly "ignore" sheet holes, folded corners or other surface and edge irregularities so as to eliminate sheet and edge sensing false alarms. Elaborate computer programs have been developed to utilize "spot" sensors, which may employ one or two light beams in close proximity to each other generally positioned at a corner of a passing sheet, as well as other sensor configurations that are only able to provide information from a small cross sectional area. In many situations (such as edge detection in predrilled paper sheets), such programs must use such corner/spot sensors to "predict" (rather than know) where the lead edge and/or trail edge of a sheet may lie. For example, when a spot or corner sensor is first triggered by the lead edge of a sheet, the program may be set so as to permit at least 8 inches (20cm) of an 8 1/2 inch (21.3cm) sheet (moving at a constant or variable velocity) to pass by the spot sensor before the program will use the spot sensor to "look" for the trail edge of the sheet. Even so, when predrilled or otherwise nonstandard sheets (size, hole placement, etc.) are run through the machine, the spot sensor (which may be able to ignore standard size holes in the usual places) may mistake a nonstandard hole near an edge as the edge itself, disrupting the timing, registration, or other essential processes being carried out by the machine. As such, when spot sensors are used to control drive motors or to monitor sheet timing, directing, registration and/or aligning mechanisms, their use can become quite problematic. Due to these and other drawbacks, known spot and corner sensors generally inhibit the ability of automated machines and systems to correctly deal with difficult or unusual situations which may not have been anticipated by their designers. Whether the situation involves nonstandard sheet holes, edges or dimensions, or sheet skew, a machine using spot sensors may be unable to adequately recognize and deal with the situation. Although it is known to utilize an array of spot sensors, such an array must be carefully arranged and tuned so as to cooperatively sense the desired condition. Such arrays of spot sensors may add complexity, expense, power consumption, undesirable heat, failure points and otherwise undesirably degrade the reliability and capability of the machine. DE-A-2 321 412 discloses a device for detecting the presence of an object (sheet), in which a light beam from a source is reflected off planar walls before being detected at a detector. In certain embodiments, plural beams travel in paths having the form of a repeating (standing) wave. In accordance with one aspect of the present invention, there is provided an apparatus for detecting a sheet moving along a path, comprising: an emitter adapted to project a light beam; a detector adapted to transmit a signal in response to receiving the light beam; and means, including a reflector for reflecting the light beam between said emitter and said detector along at least one light path which intersects the sheet path, said detector transmitting a signal indicating the absence of the sheet in response to receiving the light beam and indicating the presence of the sheet in response to the sheet intersecting the light path, characterised by including means providing a plurality of light path portions having different angles of incidence with said reflecting means, said means providing a plurality of light path portions comprising means for changing the light path such that the light beam is projected so as to be incident upon the reflector at an angle different from a previous angle of incidence upon the reflector. Preferably, said emitter is selected from the group consisting of an infra red light source. an ultraviolet light source, a laser light source, and a white light source. Preferably, said detector is selected from the group consisting of a photoreceptor, a photoresistor, a thermistor and a thermal sensor. The invention further provides an apparatus according to claim 11 of the appended claims. Other aspects and features of the present invention will become apparent as the following description proceeds and upon reference to the drawings, in which: Figure 1 is a schematic elevational view showing a sheet entry slot and sheet sensor arrangement in accordance with the present invention; Figure 2 is a schematic elevational view showing another embodiment of a sheet entry slot and sensor in accordance with the present invention; Figure 3 is a schematic elevational view showing another embodiment of a sheet entry slot and sensor in accordance with the present invention; Figure 4 is a schematic elevational view showing another embodiment of a sheet entry slot and sensor in accordance with the present invention; Figure 5 is a schematic elevational view showing another embodiment of a sheet entry slot and sensor in accordance with the present invention; Figure 6 is a schematic elevational view showing another embodiment of a sheet entry slot and sensor in accordance with the present invention; Figure 7 is a schematic elevational view showing another embodiment of a sheet entry slot and sensor in accordance with the present invention; Figure 8 is a perspective view showing a sheet sensor of the present invention; Figure 9 is an overhead view, partially in section, showing a sheet feeding system utilizing sensors of the present invention; Figure 10 is an overhead view, partially in section, showing a sheet feeding system utilizing sensors of the present invention; and Fig. 11 is a schematic elevational view showing an exemplary document copier or printer having sheet feeding, handling and stacking systems as may incorporate sensors of the present invention therein. For a general understanding of the features of the present invention, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to identify identical elements. Figure 11 schematically depicts an electrophotographic copying and/or printing machine incorporating the features of the present invention therein. For conciseness, a detailed dexcription of the machine of Fig. 11 is omitted from the present disclosure. For further information, reference is made to USSN 07/053,745, a copy of which was filed with the present application. It will become evident from the following discussion that the sensors of the present invention may be employed in a wide variety of devices and are not specifically limited in application to the particular embodiments depicted herein. Referring to Fig. 11, sensors of the present invention at points 91, 93 and at other locations in document handling unit 26 can notify the operator of malfunctions, or automatically take corrective actions. In addition, sensors of the present invention at point 75 and at other locations in transfer station D can insure the timing of the image transfer to the copy sheet is properly affected, can notify the operator of malfunctions, or automatically take corrective actions. Sensor 75 and other sensors of the present invention can monitor sheet presence, skew, alignment, trial edge, lead edge and other aspects of sheet handling. Also, sensors of the present invention at positions 57, 61, 63, 65 and at other locations can cooperatively insure duplex copying or bypass timing is properly affected, can notify the operator of malfunctions, or automatically take corrective actions. Further, sensors of the present invention at positions 65, 71, 73 and at other locations can cooperatively insure sheet feeding and timing is properly carried out, can notify the operator of malfunctions, or automatically take corrective actions. A network of optic fibers, wires or other means may be used to centralize information from sensors positioned throughout the machine; a sensor module 99 may centralize light emitting, light sensing, input processing, timing and other sensor functions in a reliable, efficient, cost effective and easily serviced or replaced pullout chassis. One skilled in the art will appreciate that a sensor of the present invention may be used at position 93 and in other circumstances in connection with high capacity feeder 76 in determining the absence, presence edge location and/or alignment of a sheet at a selected location. A sensor of the present invention at or near position 79 may also be used to inform a control processor when the desired sheet or sheets have cleared bypass deflector gate 35, and may signal the rotation of the disk slot without depending on timing or a constant sheet velocity. Figures 1-10 show various embodiments of the sheet sensor as may be employed with a copier or printer with sheet feeding, handling and stacking system. Figure 1 shows a sensor 100 having a lower reflective surface 101 and an upper reflective surface 102. Emitter 103 provides the light that is bidirectionally directed via lenses 104 and 105 onto lower reflective surface 101. Sheet presence is detected when light emitted from lens 104 is broken by a sheet or object passing between lower reflective surface 101 and an upper reflective surface 102. If no sheet interrupts the path of the beam, it is returned by mirror 106 to receptor 110 adjacent to emitter 104. Likewise, the beam of light emitted from lens 105 is broken by a sheet or object passing between lower reflective surface 101 and an upper reflective surface 102, it is returned by mirror 107 to receptor 111 adjacent to emitter 105. Preferably, in this and other embodiments of the sensor of the present invention, a well focused beam of light is used, so that any light reflected from the sheet will cause the reflector relayed or "zig-zagging" beam of light to miss mirrors 106 and 107, thereby avoiding a false indication. In that no receptors or other operative surfaces are disposed on lower reflective surface 101, there are no recesses or areas for dust and debris to clog or foul in the sensor; rather, the sliding of sheets over lower reflective surface 101 will continuously clean the reflective surface. Likewise, in that receptors 110 and 111 are (preferably) sealed from dust, and are disposed in upper reflective surface 102, sensor clogging or other malfunction is prevented. Preferably, the light emitted from lens 104 is emitted at a different angle (x) towards lower reflective surface 101 than the light emitted from lens 105 (such as an angle y), so as to reduce if not eliminate the possibility of symmetrical holes in the sheet passing through sensor 100 from falsely indicating the lead or trail edge of that sheet. If receptors 110 and 111 do not simultaneously receive light or are not simultaneously blocked from receiving light, a sheet skew condition is indicated. Figure 2 shows a sensor 115 having a lower reflective surface 116 and an upper reflective surface 117. Emitter 118 provides the light that is directed onto lower reflective surface 116; interim reflectors 119 and 120 change the angle of the beam as shown, to reduce if not eliminate the possibility of symmetrical holes in the sheet passing through sensor 115 from falsely indicating the lead or trail edge of that sheet. The beam of light emitted from emitter 118 is broken by a sheet or object passing between lower reflective surface 116 and an upper reflective surface 117. Likewise, in that receptor 121 is sealed from dust and is disposed in upper reflective surface 117 so as to prevent sensor clogging or other malfunction. If no sheet interrupts the path of the beam, it is relayed to receptor 121. Figure 3 shows a sensor 130 having a lower reflective surface 132 and an upper surface 136 having arrays 138 and 139 of receptors. Emitter 134 provides the light that is multidirectionally directed onto lower reflective surface 132. The beam of light emitted from emitter 134 is broken by a sheet or object passing between lower reflective surface 132 and an upper surface 136. If no sheet interrupts the paths of the beam, it is returned to receptors 138 and 139. The light from emitter 134 is projected at a different angles towards lower reflective surface 132 so as to reduce if not eliminate the possibility of symmetrical holes in the sheet passing through sensor 100 from falsely indicating the lead or trail edge of that sheet. If receptor arrays 138 and 139 do not simultaneously receive light or are not simultaneously blocked from receiving light, a sheet skew condition is indicated. Figure 4 shows a sensor 140 having a lower reflective surface 141 and an upper reflective surface 142. Emitter 143 provides provides the light that is bidirectionally directed via lenses 144 and 145 onto lower reflective surface 141. The beam of light emitted from lens 144 is broken by a sheet or object passing between lower reflective surface 141 and upper surface 142. Interim reflector 146 changes the angle of the beam as shown, to reduce if not eliminate the possibility of symmetrical holes in the sheet passing through sensor 140 from falsely indicating the lead or trail edge of that sheet. If no sheet interrupts the path of the beam, it is returned to receptor 147. The light from lens 145 to mirror 148, and down onto lower reflective surface 141; depending on which receptor 149 are blocked from light by the sheet, the width of the sheet passing through sensor 140 is known. A second sensor 140' (not shown) may be positioned at a right angle to sensor 140, relative to the plane of a passing sheet (see sensors 130 and 130' as shown in Figure 9). Sensor 140' would be open on the ends, so as to permit a sheet to pass lengthwise between surfaces 141' and 142'; light from shared emitter 143 is relayed by lens 145' to mirror 148' and down onto lower reflective surface 141'; depending on which receptor 149' are blocked from light by the sheet, the length of the sheet passing through sensor 140' is known. If receptors 149 and receptor 147 do not simultaneously receive light or are not simultaneously blocked from receiving light, a sheet skew condition is indicated. Figure 5 shows a sensor 150 having a lower surface 152 and upper reflective surfaces 153 and 154. Fiber optic member 156 directs light toward upper reflective surface 153 via raised lens 156 and to second raised lens 162, where it is directed towards the upper reflective surface 154. The light in fiber optic member 156 may be supplied from a central location, along with the light to other sensors positioning throughout the copying or printing machine. Illumination from a "power on" or other light source that already serves another purpose in the machine may provide the required light for sensor 150. The necessary light transmitted through fiber optic member 156 may likewise be obtained from an infrared light source, an ultraviolet light source, a laser light source, a white light source, a colored light source, a fiber optic light source and/or even utilize ambient room light. The light sources described above may likewise provide light to the other sensors shown and described in conjunction with Figures 1-4 and 6-10 herein. Functional Raised lenses 157 and 162 direct light towards upper reflective surfaces 153 and 154 at differing angles and along differing paths; raised lower reflectors 163 cooperate with upper reflective surfaces 153 and 154 in directing light to raised receptors 160 and 164. Light received by raised receptors 160 and 164 may be relayed to a central location by fiber optic members 161 and 165, respectively, where it is sensed by a photoreceptor, photoresistor, thermistor, thermal sensor or other sensing means. A sheet or object passing between upper reflective surfaces 153 and 154 and lower surface 152 will interrupt the beam of light; if no sheet interrupts the paths of the beam, it is returned to raised receptors 160 and 164. If raised receptors 160 and 164 are not simultaneously blocked from receiving light, a sheet skew condition is indicated. The sliding of sheets over raised reflective surfaces 163, raised lenses 156, 162 and raised receptors 160, 164 provides a continuous cleaning action, preventing dust and debris from interfering with the proper operation of sensor 150. Likewise, false reflections and indications are prevented by the non-raised areas of lower surface 152 that surround the aforementioned raised lenses, reflectors and/or receptors. Further, the uses that can be made of the optic fiber members, emitter lenses, reflectors and/or receptors described in conjunction with Figures 6 and 8 may be used in conjunction with other embodiments of the sensor of the present invention. Likewise, the sensors shown and described in conjunction with Figures 1-4 and 6-10 herein may be wired and/or optically connected to centralized and/or modularized emitter and receptor assemblies capable of servicing a plurality of sensors throughout the copier or printer machine. Such centralized sensor assemblies with modularized, pull-out chassis could reduce costs while increasing manufacturing, operational and maintenance efficiency. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variations of optic fiber members, lenses, reflectors and/or receptor networks that fall within the scope of the embodiments claimed. Figure 6 shows a sensor 170 having a lower reflective surface 172 and an upper reflective surface 174. Emitter 176 directs light towards lower reflective surface, so that after being passed back and forth between the upper and lower reflective surfaces it is sensed by receptor 178, provided it is not interrupted by a sheet or object passing between them. Mount 177 may be included on certain embodiments, and may permit the angle of the beam from emitter 176 to be varied, further reducing if not eliminating the possibility that symmetrical holes in a sheet passing through sensor 170 might falsely indicate a lead or trail edge. The distance or gap between lower reflective surface 172 and upper reflective surface 174 may be altered, so as to permit the sensor to detect a stack of sheets, or otherwise permit desired additional sheet clearance. By adjusting emitter 176 in mount 177, or by changing the gap between lower reflective surface 172 and upper reflective surface 174 so as to permit the beam of uninterrupted light to focus on receptor 178, sensor 170 may be employed in a variety of locations in a copier or printer. Light emitted may also be of a nature so that receptor 178 or a secondary receptor positioned in the path of the reflected beam of light, if provided with spectral analysis capability, can determine the color of the sheet. This capability may prevent the accidental use of a colored toner with a sheet of the same color, may automatically adjust toner level(s) and the like to ,achieve a higher quality print or copy image, or may provide other operational advantage. Figure 7 shows a sensor 180 having an upper reflective surface 182 and a lower reflective surface 183. Emitter 184 directs beams X and X' of light from lenses 186 and 188; the beams may be identical or of differing spectral characteristics to permit receptor 190 to differentiate between them. Light is directed towards lower reflective surface 184, and after being transmitted back and forth between the upper and lower reflective surfaces (except as it is interrupted by a sheet resting or passing between them) is sensed by receptor 190. Figure 8 is a perspective view show another embodiment of the present invention. Sensor 190 includes raised reflective surfaces 192 between recessed areas 193 on the lower surface of sensor 190. Emitter 196 directs parallel beams of light towards the lower reflective surfaces 192, and after being passed back and forth between them and upper reflective surface 194 (except as interrupted by a sheet passing between them) is sensed by receptor 198. The sliding of sheets in direction D over raised reflective surfaces 192 provides a continuous cleaning action; dust and debris is trapped or directed away from the sensor by recesses 193. False reflections are also prevented by recesses 193, and by the narrow width of reflective surfaces 192, which only permit properly focused light to reach receptor 198. Figure 9 shows a sheet registration system which utilizes sensor(s) of the present invention to provide an active registration system which sense document position and operates to correct that positioning if necessary. A sheet deskewing arrangement may be provided with a sensor 130 having open ends (and/or a second sensor 130') as described in conjunction with Figure 3, or other suitable multidetector sensor described herein. Each sensor may be arranged along a sheet path, operably coupled with selectably controllable drive motors each driving a driving nip in a nip roll pair, to correct skew sensed by the sensors. The sensors detect when a lead and/or side edge of a sheet passes thereby; a difference in the timed sensing of individual receptors (as described in conjunction with Figures 1-8 above) will indicate sheet skew, and the two motors are driven in accordance with the difference to accelerate or decelerate a side portion of the sheet, thereby rotating the sheet to bring the sheet into proper alignment and registration. Edge stop 205, adjacent to sensor 130 (and/or receptor arrays 138 and 139 as shown in Figure 3) can monitor and prevent lateral shift of sheet 200, and otherwise insure uniform positioning and feeding of each sheet for subsequent operations in the copier or printer. Figure 9 shows a sheet registration, alignment and/or deskewing system that utilizes one or more sensors of the present invention. Sheet 200 is fed by drive rolls (not shown) in direction D toward the registration roll sets 210 and 212, which has been initialized at the approximate centerline of the paper path. Leading edge 202 of sheet 200 initially contacts one side of the registration rolls, in this case, roll pair 214 and 216, which rolls are initially in a stopped position. Stepper motor 226 drives belt 230 on roller axles 222 and 234; likewise, stepper motor 228 drives belt 232 on roller axles 224 and 236. Cooperatively, these stepper motors manipulate the positioning of sheet 200 according to the information provided by sensor(s) 130 and/or 130'. A controller (nor shown) counts the steps taken by the stepper motor; an appropriate edge 201 and/or 202 is sensed by the sensors 130 and/or 130'. In this manner, sheet 200 continues to be driven forward by drive rolls in the direction of arrow D until a side edge 201 fully engages sensor 130, and/or until lead edge of sheet 202 fully engages sensor 130'; according to the information provided by the sensor as to the skew of the sheet, sheet 200 is rotated until sheet leading edge 202 is aligned in both registration roll pairs 210 and 212. Once the sheet 200 is in the proper registration/alignment position, it continues to be driven in the direction of arrow D by unison rotation of rollers 214 and 215. Depending on which individual receptor(s) in arrays 138 and 139 are blocked from light by sheet 200 (see Fig. 3), sensors 130 and or 130' can determine whether the sheet is laterally shifted and/or skewed. Preferably, stepper motors 226 and 228 manipulate side edge 202 to engage sensor 130; edge stop 205 may assist in insuring that side edge 202 is uniformly positioned for subsequent operations in the copier or printer. Registration roll sets 210 and 212 then prepare to accept the next sheet. The path of sheet 200 in Figure 9 may be provided with one or more sensors 130 and 130' as shown and described in accordance with Figure 3, or a combination of other sheet sensors in place of sensors 130 and 130' such as those shown and described above in accordance with Figures 1-2 and 4-10. Sensors may be spaced apart on a line which is substantially perpendicular to the path of travel of sheet 200, at right angles, or otherwise according to the desired function, as well as to accommodate the available space in the copier or printer. Figure 10 shows a schematic representation of another sheet feed system that may utilize one or more sensors of the present invention. Such an arrangement incorporating the sensor(s) of the present invention finds advantageous use in any application where the alignment and/or presence of discrete sheets of material must be fed, registered or otherwise moved through a copier or printer. Uses include, for example, a primary, auxiliary or high volume copy sheet feed or stack setup, a recirculating document handler, and numerous others. Sheet 250, with predrilled holes 252, torn lead edge 254, torn trail edge 256 and folded corner 258 is shown being moved in Figure 10. Sheet 250 is fed into feed rollers 260 powered by motor 262 in direction D; despite the irregular edges and unusual hole configuration of sheet 250, sensor 140 is able to correctly sense (depending on which receptors of receptor array 149 are blocked from light) the width and presence of sheet 250; likewise, due to the changes the angle of the light beam (see Figure 4), the lead an/or trail edge of sheet 250 is properly indicated by sensor 140. Sheet 250 is fed so that edge 251 is adjacent to edge stop 264. As sheet 250 interrupts the path of the beam, if receptor array 149 and receptor 147 do not simultaneously receive light or are not simultaneously blocked from receiving light, a sheet skew condition is indicated.

Claims: An apparatus (115;140;170) for detecting a sheet moving along a path, comprising: an emitter (118;143;176) adapted to project a light beam; a detector (121;147;178) adapted to transmit a signal in response to receiving the light beam; and means (116,117,141,142;172,174), including a reflector (116;141;172) for reflecting the light beam between said emitter and said detector along at least one light path which intersects the sheet path, said detector transmitting a signal indicating the absence of the sheet in response to receiving the light beam and indicating the presence of the sheet in response to the sheet intersecting the light path, characterised by including means providing a plurality of light path portions having different angles of incidence with said reflecting means, said means providing a plurality of light path portions comprising means (119,120;146;177) for changing the light path such that the light beam is projected so as to be incident upon the reflector (116;141;172) at an angle different from a previous angle of incidence upon the reflector. The apparatus of claim 1, wherein said reflecting means comprises a plurality of substantially planar surfaces (116,117;141,142;172,174) adapted to cooperatively focus the light beam along the light path from said emitter to said detector. The apparatus of claim 2, wherein said plurality of planar surfaces comprise planar surfaces positioned to contact the sheet moving along the sheet path. The apparatus of claim 2 or 3, wherein said plurality of planar surfaces comprise at least two substantially planar surfaces mounted opposed from one another defining a slot therebetween through which the sheet advances. The apparatus of any of the preceding claims, further comprising a processor (99) receiving the signal from said detector to sense an edge of the sheet, the processor preferably being adapted to discriminate between an aperture in the sheet and the absence of the sheet. The apparatus of any of the preceding claims, wherein said means (119,120;146;177) providing a plurality of light path portions comprising a second reflector means (146;162), wherein said second reflector means is positioned so as to alter an angle of incidence of the light being reflected to said detector. The apparatus of any of the preceding claims, wherein said emitter projects the light beam along a plurality of light paths. The apparatus of any of the preceding claims, further comprising a second light detector (111;139;149;164). The apparatus of any of the preceding claims, further comprising: a second detector (111;139;149) further adapted to transmit a second signal in response to receiving a second light beam; and a processor (99) capable of accepting input from said first mentioned detector and said second detector so as to detect a length of at least one edge of said sheet. The apparatus of any of the preceding claims, wherein said detector detects a color of the sheet. An apparatus for detecting a sheet moving along a path, comprising: an emitter (103;134;143;157;176;184) adapted to emit a plurality of light beams; detecting means (110,111; 138,139; 147,149; 160,164; 178; 190) adapted to transmit a signal in response to receiving a light beam; means (101,102;132,136;141,142,148;153,154,162;172,174;182,183) for reflecting the light beams between said emitter and said detecting means along light paths which intersect the sheet path, said detecting means transmitting a signal indicating the absence of the sheet in response to receiving the light beam and indicating the presence of the sheet in response to the sheet intersecting the light path, characterised by including means providing a plurality of light path portions having different angles of incidence with said reflecting means, said means providing a plurality of light path portions comprising means (104,105;134;144,145,157,162;177;186,188) for projecting each of the plurality of light beams at a different angle so as to intersect the sheet path at a different location, wherein said detecting means comprises a plurality of detectors (110,111;138,139;147,149;160,164), each detector being positioned so as to receive a respective light beam, or said detecting means comprises a single detector (178;190) and said different angles are predetermined so that the light beams are received by said single detector.

Inventors: RABJOHNS DOUGLAS T, ROSCOE GARY W, RABJOHNS, DOUGLAS T., ROSCOE, GARY W.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1994
Main classification: G03G15
IPC codes: G01N21, G03G15, B65H7